It is known to use a mixture of oxygen-containing material, such as Ni-NiO mixture, Cd-CdO mixture or Zn-ZnO mixture, as an oxygen ion reference source of the oxygen sensor. Usually, the mixture is placed in a cavity formed in the electrolyte so as to electrically connect one surface defining the cavity to an electrode member immersed in the mixture, so that an electromotive force (EMF) is developed between the other surface of the electrolyte and the electrode member with a minimum electrical resistance. In such a conventional oxygen sensor, however, the oxygen-containing mixture is powdery (or sometimes a paste), so that a tight setting of the mass of the mixture in the cavity of the electrolyte is not expected because of its fluidity. Usually, some biasing means pressing the mass on the surface defining the cavity is employed for allowing the mass to maintain its original form inducing the assured electrical connection between the surface and the electrode member. However, since there is a limitation in ability to compress the powdery mixture of the oxygen-containing material, it sometimes happens that the mixture initially fixed becomes loose due to vibrations and heat hysteresis applied thereto during its use with a result that the electrical connection prepared by it becomes worse with increased electrical resistance. In addition to this, in the prior art oxygen sensor mentioned above, some sealing means is required to prevent the powdery mixture of the oxygen-containing material from being lost through any slit formed in the sensor. This induces increased production costs of the sensor. Consequently, the prior art oxygen sensor mentioned above is somewhat impractical due to the drawbacks.